1. Field of the Invention
The present invention relates to a display apparatus for displaying an image on a screen.
2. Description of the Prior Art
FIG. 1 shows an example of a known projection type raster-scan system display apparatus of utilizing laser beams to display an image on a projection screen.
Referring to FIG. 1, laser light sources 1R, 1G and 1B generate red, green and blue laser beams, respectively. The red, green and blue laser beams emitted from these laser light sources 1R, 1G and 1B are respectively supplied to optical modulators 2R, 2G and 2B. Red, green and blue primary color signals R, G and B are respectively supplied to the optical modulators 2R, 2G and 2B for modulating laser beams, wherein the red, green and blue laser beams are modulated in intensity (luminous intensity) by the color signals R, G and B, respectively.
The laser beams, modulated in intensity by the optical modulators 2R, 2G and 2B, are respectively supplied through lenses 3R, 3G and 3B to dichroic mirrors 4R, 4G and 4B, in which they are mixed to provide a single display laser beam Lw. Laser beam Lw is supplied to a polygon mirror 11 for horizontal deflection. The above-mentioned lenses 3R, 3G and 3B are used to adjust beam spots of the laser beams. Dichroic mirrors 4R, 4G and 4B are used to reflect laser beams of corresponding colors and to pass laser beams of different colors therethrough.
The polygon mirror 11 is formed as, for example, a regular icosipentahedron cylinder and the surface thereof is finished by a mirror-grinding process. A motor 12 is adapted to rotate the polygon mirror 11 at a revolution frequency of 1/25 of the horizontal frequency of each of the signals R, G and B in synchronism with a horizontal synchronizing pulse so that the laser beam Lw is horizontally deflected 25 times per revolution of the polygon mirror 11.
The laser beam Lw is collimated by a first relay lens (i.e. cylindrical lens) 13, and the collimated beam is supplied to a second cylindrical lens 14 having the same focal length as that of the first cylindrical lens 13, in which a reflection point of the polygon mirror 11 is focused at the focus position of the second cylindrical lens 14. A galvano mirror 15 is located at the above-mentioned focus position to vertically deflect the laser beam. A galvano motor 16 is adapted to vibrate the galvano mirror 15 at a vertical frequency of each of the signals R, G and B in synchronism with a vertical synchronizing pulse. Therefore, the laser beam Lw is vertically deflected by the galvano mirror 15 in the direction at a right angle to the direction in which the polygon mirror 11 horizontally deflects the laser beam Lw.
The laser beam Lw, horizontally and vertically deflected, is projected through a projection lens 17 on the rear surface of a flat screen 18. The projection lens 17 is employed to increase the resolution of the laser beam Lw by decreasing the beam spot of the laser beam Lw on the screen 18. The screen 18 is of such a type that an image projected on the rear surface thereof can be seen from its front side. Thus, a color image of signals R, G and B is displayed on the screen 18, when it is seen from its front surface.
While the above-mentioned polygon mirror 11 is rotated at an equal angular velocity, assuming the screen 18, for example, is flat, the laser beam Lw does not horizontally scan the screen 18 at an equal velocity. As a result, a rotation angle .theta. of the polygon mirror 11 and the horizontal scanning position of the laser beam Lw satisfy a relationship expressed by tan .theta., causing linearity in the horizontal direction of the image displayed on the screen 18 to deteriorate particularly at the left and right end portions of the screen 18.
In order to solve the above-mentioned problem, it has been proposed to correct linearity in the horizontal direction of the image displayed on the screen 18 by controlling the rotary phase of the horizontal deflection polygon mirror 11 during the horizontal scanning period, similarly to a television receiver in which linearity in the horizontal direction of an image is corrected by correcting the waveform of a horizontal deflection signal.
The polygon mirror 11 must operate with high precision. Therefore, the polygon mirror 11 is made of a material such as aluminum and the like and is finished by a mirror-grinding process. Accordingly, the polygon mirror 11 has a large mass and its rotation frequency is 1/25 of the horizontal frequency of the signals R, G and B, i.e. the rotation speed is as high as about 630 r.p.s. Thus, in practice, even if the polygon mirror 11 could be rotated in synchronism with the horizontal synchronizing pulse of the signals R, G and B, the rotary phase of the polygon mirror 11 could not be adequately controlled during one horizontal scanning period in practice. It is therefore impossible to correct the linearity of the image displayed on the screen 18 by controlling the rotary phase of the polygon mirror 11 during the horizontal scanning period.
It is to be noted that when the scanning screen is as small as that of a laser-type printer, the non-linearity in the horizontal direction of the image can be corrected by a correcting lens. However, when the screen is large as in a laser display apparatus, it is impossible to correct the non-linearity in the horizontal direction of the image on the screen by using a correcting lens.